1. Field of the Invention
This invention relates to a pneumatic tire having substantial parallelogram block patterns. More specifically, it relates to a pneumatic tire capable of preventing irregular wear at the wear early stage and obtaining sufficient drainage and traction performance at the wear end stage.
2. Description of the Background
The following description sets forth the inventor's knowledge and should not be construed as an admission that the description constitutes prior art.
In pneumatic tires having substantial parallelogram block patterns, the block is smaller in rigidity at the acute corner portion than at the obtuse corner portion, causing irregular wear.
In order to prevent such irregular wear, the groove side of the block was so designed that the inclination angle of the groove side at the acute corner portion was set to be larger than that of the groove side at the obtuse corner portion. Consequently, the rigidity at the acute corner portion was enhanced and therefore the rigidity of the block was equalized at these corner portions, preventing irregular wear of the block. For example, a pneumatic tire disclosed in Japanese Examined Laid-open Patent Publication H7-155569 is known.
In such a conventional pneumatic tire, the inclination angle of the groove side of the block is different between at the obtuse corner portion and at the acute corner portion. Therefore, the extending direction of the main groove at the tread surface level and that at the groove bottom level are different.
For example, FIG. 1 shows a pattern of a pneumatic tire in which main groove 101 extends substantially in a circumferential direction R of the tire and transverse groove 102 extends obliquely with respect to the widthwise direction A of the tire and connects the main grooves 101. Thus, substantial parallelogram blocks 103 are formed. In FIG. 1, the groove side 104b at the side of the obtuse corner portion 103b of the block 103 is substantially vertical, while the groove side 104a at the side of the block acute corner portion 103a is inclined. Since the traction performance can be improved in cases where the main grooves 101 extend obliquely with respect to the circumferential direction R of the tire, the extending direction 101a of the main groove 101 on a tread surface level is formed obliquely with respect to the circumferential direction R of the tire. As the tread wear increases, however, the extending direction 101b of the main groove 101 comes closer to the circumferential direction R of the tire due to the inclination of the groove side 104. As a result, the traction performance deteriorates at the wear end stage.
On the other hand, in another pattern of a pneumatic tire shown in FIG. 2, the extending direction 201a of the main groove 201 at the tread surface level is set to be substantially coincide with the circumferential direction R of the tire. Since the groove side 204b at the side of the obtuse corner portion 203b of the block is substantially vertical and the groove side 204a at side of the acute corner portion 203a of the block is inclined, the main groove 201 extends obliquely with respect to the circumferential direction R of the tire at the groove bottom level. This causes the width of the main groove 201 to be decreased as the wear increases, decreasing the width of the main groove 201 and deteriorating the drainage performance as the wear increases. As described above, it has been difficult to keep both the good traction performance and the good drainage performance at the same time at the wear end stage.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.